1. Field of the Invention
This invention relates to a digital wireless communication system operating in a microwave band, such as the 2.4 to 2.5 GHz ISM (Industrial Scientific and Medical) Band. Such a system is useful, for example, for communicating many types of data, such as music data, video camera data, digital image data, web page data, etc., at high data rates; e.g. rates equal and greater than 10 Mega bits per second (Mbps).
2. Description of the Related Art
With the advent of digital data transmission applications such as short messaging service, email, digital music, video camera, digital photography, etc., the wireless transmission of digital data, for example between a transmitting device like a digital camera, computer or a music source and a receiving device such as a personal computer or other appliance, such as a printer, personal digital assistant (PDA) or music player, has become a desirable feature. High data rate transmissions are very desirable because digital images, music, web pages, etc. include a large amount of data and short transmission times are needed. Short transmission times result in shorter wait times while an image, a song, page, etc. is being transferred from a source to a receiver and in reduced battery power consumption. An additional desirable feature is being able to communicate at distances greater than 300 feet, and with new power licenses, and also to communicate through walls. These features would enable digital appliances, such as cameras, PDAs, or music players, to also communicate with associated equipment when the equipment is located inside a retail store, a home, etc. and the appliance is being operated, for example, from inside a nearby automobile. One additional useful feature is that the wireless communication be unlicensed and worldwide such that the digital appliance could be taken on vacation and used in any country in the world without airtime costs.
One way to send digital data without wires to a digital appliance is through the use of infrared technology. Similar in the way a remote control for a television or VCR works; digital images can be sent in a direct line of sight over short distances. Most current infrared technology that use the IrDA 1.1 standard are limited to data rates of about 4 Mbps. This technology is inexpensive, but does not work through walls and has maximum operation range less than 30 feet. Another way to send data without wires is through the use of radio waves. A use of radio waves to perform image communication is described in U.S. Pat. No. 5,806,005, issued Sep. 8, 1998 to Hull et al, entitled, “Wireless Image Transfer From A Digital Still Video Camera to a Network Computer”. This patent demonstrates a potential solution of moving digital images using a cellular telephone transmitter. Unfortunately this technique has very slow data rates due to the use of the cellular telephone. The cellular phone is physically too large to be incorporated into the body of the digital appliance and results in an expensive system to both purchase and operate. The user of the system would not only need to purchase both the digital appliance, connecting wire and cellular phone but also pay by the minute for the air time (talk time) for using the cell phone. Worldwide use would be restricted and battery consumption would be high.
Therefore an improvement is needed for enabling digital cameras to communicate without wires. The most important factor in determining the usefulness of the wireless system would be selecting the frequency band of operation. There are several radio frequency bands that could be used for such type of transmissions. One of the most attractive communication bands for such a digital communication system is the 2.4 to 2.5 GHz ISM Band since the band is unlicensed and the only wide band available internationally to date. However, a major problem exists with the use of this band for wireless communication. The band is allocated to devices that produce periodic noise, such as microwave ovens and magnetron driven lightning systems, which generate a great deal of noise in the band.
Radio frequency (RF) transmissions in the 2.4 to 2.5 GHz ISM Band have historically had to deal with the presence of man-made noise from microwave ovens and presently proposed magnetron driven lighting systems that predominately operate in the center of this band at 2.45 GHz. The noise emanates from the oven or lighting structure by leakage through the devices enclosure. The leakage noise has a radiated output power approximately 20 to 30 dB greater in strength than that allowed by the FCC for operation of Part 15 non spread spectrum radios (i.e. approx. 1 milliwatt). With over 200 million microwave ovens in use throughout the world, they are currently the greatest and most significant source of noise in this band. Some examples of locations where it would be desirable to transmit data in the presence of microwave oven noise are in the home (particularly in the kitchen), or in a supermarket or retail store where a photo kiosk or digital minilab may be located near a microwave oven.
Another problem involved with microwave devices is the irregular and somewhat random periods in which they are operated, and their duty cycle mode of operation. When operating at full power a typical microwave oven has a periodic operating pattern of on/off cycles or duty cycles associated with the 60 Hz power source used for these devices. In addition, when an oven is operating at less than full power the oven operates with a super duty cycle. For example, in the defrost mode an oven might be on for 2 seconds and off for 8 seconds for a duty cycle of 20%. This super duty cycle constitutes another source of noise and surrounds the 50% duty cycle of the AC power division periodic noise.
Other sources of noise in the microwave band that have cycles and super cycles include rotating antenna microwave radars, such as those used for weather, and other microwave devices such as proposed lighting systems. With the advent of microwave lighting systems in venues where photography takes place (such as stadiums and museums), the problem becomes even more relevant.